1. Field of the Invention
The present invention relates to a method of and an apparatus for protecting data on a storage medium, which are intended to protect the data to be recorded on the storage medium by encrypting the data with a password, and to the storage medium thereof in an information processing appliance.
2. Description of the Related Art
A storage device utilizing an optical disk, a magnetic disk and an IC card etc, is utilized for a variety of information processing appliances such as a computer, a word processor and an electronic book etc. Information such as information related to privacy and confidential information in terms of duties, which should not be known by persons other than an owner, might be written to this type of storage device. It is required that the data be encrypted in order to make such information unknown by others.
FIG. 15 is an explanatory diagram showing the prior art.
A password is set on a storage medium 90 such as an optical disk etc or on a storage device. When writing the data, an encrypting unit 91 encrypts the data with the password, and the encrypted data is written to the storage medium 90. Further, when reading the data, a decoding unit 92 decodes the data on the storage medium 90 with a password.
Thus, a data confidentiality can be kept by encrypting the data. In this connection, there has hitherto been a method of setting one password on the whole storage medium. There also has been a method of setting passwords different based on a file unit of the storage medium.
First, as cipher texts defined as samples or combinations of the cipher texts with unencrypted plain texts become larger in quantity, the decryption by a decipherer becomes easier. As a result, the same plain text is encrypted with the same password. Therefore, when encrypted directly with the same password, a statistic characteristic of the cipher text reflects in a statistic character of the plain text. Accordingly, a conventional method of encrypting with the same password on the storage medium presents such a problem that if a volume of the cipher texts is large enough to make a statistic process executable, the characteristics of the plain texts can be presumed easily.
Second, data is stored in a large capacity storage medium such as an optical disk etc. A portion of the data such as a directory portion is structured in a fixed format. A problem peculiar to the conventional method of encrypting with the same password on the storage medium is that the password is presumed by analyzing this portion, in which case other vital data are to be deciphered.
Third, according to the conventional method of setting the password per file, when the password of some portion is decrypted, other portions can be prevented from being decrypted. In this case, however, it is required that the different password be managed per file. This operation is troublesome and might cause a problem in which a fault such as forgetting the password and so on can easily occur.
Fourth, in the large capacity exchangeable storage medium such as an optical disk etc, it is possible to take the storage medium out and copy the storage medium. Therefore, the once-encrypted data is carried out and may be analyzed later on taking a sufficient period of time. Accordingly, the problem is that the password is easy to be presumed form the cipher text.
A fifth problem is that the data has hitherto been encrypted directly with the password, and hence, if the password is changed, the whole data are required to be re-encrypted.